28121809 has 3 divisors (see below), whose sum is σ = 28127113. Its totient is φ = 28116506.

The previous prime is 28121803. The next prime is 28121827. The reversal of 28121809 is 90812182.

It is a happy number.

The square root of 28121809 is 5303.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 90812182 = 2 ⋅45406091.

It is not a de Polignac number, because 28121809 - 2²³ = 19733201 is a prime.

It is a super-2 number, since 2×28121809² = 1581672282864962, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28121803) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 2652 + ... + 7954.

Almost surely, 2^28121809 is an apocalyptic number.

28121809 is the 5303-rd square number.

28121809 is the 2652-nd centered octagonal number.

It is an amenable number.

28121809 is a deficient number, since it is larger than the sum of its proper divisors (5304).

28121809 is an frugal number, since it uses more digits than its factorization.

28121809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10606 (or 5303 counting only the distinct ones).

The product of its (nonzero) digits is 2304, while the sum is 31.

The cubic root of 28121809 is about 304.0985981250.

Multiplying 28121809 by its product of nonzero digits (2304), we get a square (64792647936 = 254544²).

The spelling of 28121809 in words is "twenty-eight million, one hundred twenty-one thousand, eight hundred nine".